1. Field of the Invention
The present invention generally relates to laser resonator cavity configurations and more specifically, to a beam alignment sensitivity and multi-moding elimination configuration and technique for laser resonator cavities.
2. Description of Prior Art
In order to maximize the efficiency of heterodyne detection when utilizing coherent lasers, the local oscillator (LO) beam and the received laser radiation should be aligned coaxially when combined at the detector. In laser radars of the prior art this alignment has been accomplished using beam splitters. In these traditional arrangements, alignment of the beam splitters is critical and very little misalignment can be tolerated without a drastic reduction in mixing efficiency. With the relatively recent advent of solid state near infrared (NIR) coherent laser radars it has been possible to replace the beam splitters by single mode fiber optic couplers which automatically maintains alignment between the two coupled beams. This arrangement transfers the alignment point to a point where the beams are coupled into the fiber optic. This arrangement is much less sensitive to misalignments than the beam splitter arrangements, but it is not completely alignment insensitive.
Another problem encountered in coherent laser radar transmitter design is multi-moding. Heterodyne mixing efficiency is maximum for the case of the LO and received beams corresponding to the same single longitudinal-transverse mode. For a single resonator longitudinal-transverse mode there are nodal regions where little stimulated emission occurs and the gain remains high due to interference between counterpropagating waves. These nodal regions may support laser action for other resonant cavity modes. When this occurs the output of the laser is multi-mode and not useful for coherent laser radar applications.
While the prior art has reported using techniques which maximize efficiency for laser radar, none have established a basis for a specific apparatus that is dedicated to the task of resolving the particular problem at hand. What is needed in this instance is a laser resonator cavity configuration that eliminates multi-moding and where the LO and received beam are alignment insensitive.